Additionally, in Refs. [50, 51], excited state calculations were carried out and the
CPL spectra for the three molecules were computed: it was found that in the excited
state a large variation occurs in the inter-unit dihedral angle with respect to the
ground state, by an amount varying in the three molecules from ca. 30
to ca. 50
.
Still the sense of rotation of one BT unit to the other BT unit is univocally related to
chirality M or P for the molecule, which does not change from the ground to the
excited state. The latter characteristic is related to the fact that the sign of the CD
observed at the longest wavelength is the same as the sign of the unique CPL band.
Another difference from the ground to the excited state is the flattening of the
different conjugated parts in the two moieties. Similar g lum and g abs values were
observed for the oligomers of BT2-T4 and of BT2-CPDT2 [52]. It is worthwhile to
recall that, in presence of exciton couplets, the observed g abs may be influenced by
partial mutual cancellation of the two components of the couplet depending on
bandwidth and exciton wavelength separation; on the contrary, g lum is determined
by just one component of the couplet.
Fig. 10.8 (Left): ECD and CPL spectra of (S)-2,2’-bis(2,2’-bithiophene-5-yl)-3,3’-bithianaphthene (BT2-T4). (Center): ECD and CPL spectra of 4H-cyclopenta[2,1-b3:4b’]
dithiophene-3,3’-bithianaphthene (BT2-CPDT2). (Right): ECD and CPL spectra of dithieno
[3,3-b:2’,3’-d]pyrrole-3,3’-bithianaphthene (BT2-DTP2). Corresponding fluorescence spectra
have been normalized: (S,S)-(red) and (R,R)-(black) (see Refs. [50, 51])
10 Structural and Electronic Information Drawn. . .
233
CPL spectra for the three molecules were computed: it was found that in the excited
state a large variation occurs in the inter-unit dihedral angle with respect to the
ground state, by an amount varying in the three molecules from ca. 30
to ca. 50
.
Still the sense of rotation of one BT unit to the other BT unit is univocally related to
chirality M or P for the molecule, which does not change from the ground to the
excited state. The latter characteristic is related to the fact that the sign of the CD
observed at the longest wavelength is the same as the sign of the unique CPL band.
Another difference from the ground to the excited state is the flattening of the
different conjugated parts in the two moieties. Similar g lum and g abs values were
observed for the oligomers of BT2-T4 and of BT2-CPDT2 [52]. It is worthwhile to
recall that, in presence of exciton couplets, the observed g abs may be influenced by
partial mutual cancellation of the two components of the couplet depending on
bandwidth and exciton wavelength separation; on the contrary, g lum is determined
by just one component of the couplet.
Fig. 10.8 (Left): ECD and CPL spectra of (S)-2,2’-bis(2,2’-bithiophene-5-yl)-3,3’-bithianaphthene (BT2-T4). (Center): ECD and CPL spectra of 4H-cyclopenta[2,1-b3:4b’]
dithiophene-3,3’-bithianaphthene (BT2-CPDT2). (Right): ECD and CPL spectra of dithieno
[3,3-b:2’,3’-d]pyrrole-3,3’-bithianaphthene (BT2-DTP2). Corresponding fluorescence spectra
have been normalized: (S,S)-(red) and (R,R)-(black) (see Refs. [50, 51])
10 Structural and Electronic Information Drawn. . .
233